A Closer Look: Fossils

What is a fossil?

Fossilized leaf.

Fossils are either the preserved remains of ancient animals and
plants, their imprints, or the traces of their activities.
Fossils are most often preserved in sedimentary rock. However,
organisms preserved in ice, or organic matter such as amber or
tar, are
also
considered fossils if the organisms are from the prehistoric
past.

Fossilization is a rare event. The chances of a given individual
plant or animal becoming a fossil are very small. The hard
skeletons and shells of animals and hard woody plant structures
are more
likely to become fossilized than the soft tissues of organisms.
Fossilization is also more likely to occur in marine environments.
Even if
fossils
are able to form, they may not be preserved intact. For example,
in some cases, sedimentary rock that is buried and compacted
experiences temperature and pressure increases that can melt
and deform it,
changing
it to metamorphic rock. Fossils preserved in sedimentary
rock that becomes metamorphic rock are usually distorted or destroyed.

How are fossils formed?

Unaltered Preservation
When organisms become
trapped in substances such as amber, tar, or ice, their remains
can be preserved
unchanged. In some cases, the structure of the organism's
soft tissue is even
preserved.

Petrifaction
When petrifaction occurs, mineral
rich fluids slowly seep into the cells of an organism and
crystallize, displacing the
original organic matter. Petrifaction is also referred
to as permineralization. Most bone and wood fossils
are permineralized.

Replacement
When an organism is buried in
mud, the minerals that make up its hard parts can dissolve
and be replaced
by different minerals such as calcite or silica.
This process can preserve
the soft
tissue of organisms, providing important information
on ancient organismal structures.

Carbonization
This process is also referred to as coalification. Plants
are usually preserved through
this process when
bacteria decay the plant material and in doing
so release elements like hydrogen, oxygen, and nitrogen.
This reduces most
of the remaining
matter
to
carbon.

Recrystallization
Many aquatic animals
have shells composed of a form of calcium carbonate that
will change into
calcite, a more stable mineral, over millions
of years. This process
also rearranges
small calcite crystals into larger crystals,
preserving the original material but destroying its original
structure.

Authigenic Preservation
Fossilized
organisms preserved in rock can be destroyed or dissolved leaving
an
empty space in the rock where
there once was an organism. The impression that
is left behind in the rock is called an external
mold. Sometimes the inside
of a shell is
filled with sediment before the shell dissolves
or
is
destroyed. This is an internal mold. If the shell
is filled with a
new mineral, the
shell replica is called a cast.